Enzymes which are capable of hydrolyzing a preheated starch slurry include .alpha.-amylase, .beta.-amylase, glucoamylase and .alpha.-1,6-glucosidases, pullulanase and isoamylase.
Starch is composed of amylose- and amylopectin molecules. Amylose is a straight chain of numerous glucose units which are connected to each other with .alpha.-1,4-glucosidic linkages. Amylopectin is a branched molecule where at the branchpoints the glucose chains are attached to each other with an .alpha.-1,6-linkage.
.alpha.-Amylase cuts .alpha.-1,4-linkages randomly from any part of the starch molecule yielding glucose chain residues of different shapes and sizes. .beta.-Amylase acts differently--it yields maltose molecules (diglucose units) from the non-reducing ends of a starch molecule as long as it reaches a branchpoint which it cannot pass. Glucoamylase is capable of cutting both .alpha.-1,4- and .alpha.-1,6-linkages apart from certain very low molecular weight residues (2-4 glucose units which often have a branch point in the molecule), thus it yields glucose almost exclusively. Pullulanase and isoamylase cut only .alpha.-1,6-linkages.
For the liquifaction and saccharification of starch-containing slurries, as for example in brewing, distillation, and starch syrup manufacture, there are readily available various microbially produced commercial preparations such as .alpha.-amylase, glucoamylase and, also to some extent, pullulanase. (See, for example, Johnson, 1977, Industrial Enzymes-Recent Advances, Noyes Data Corp., New Jersey; Fogarty, 1983, Microbial Enzymes and Biotechnology, Applied Science Publishers, London and New York; and Godfrey & Reichelt, 1983, Industrial Enzymology, The Applications of Enzymes in Industry, Macmillan Publishers Ltd., The Nature Press, New York.)
There are also maltose-producing enzymes available. One is a so-called fungal-amylase which acts primarily like .beta.-amylase but also partly like an .alpha.-amylase. The other is a so-called diastatic malt extract made from malted, i.e. germinated, barley. In addition to .beta.-amylase, diastatic malt extract also contains substantial amounts of .alpha.-amylase and proteases.
All the above-mentioned preparations are enzyme mixtures. For several industrial users, however, a pure .beta.-amylase is more suitable. According to present knowledge, pure .beta.-amylase cannot be produced by microbes. .beta.-Amylase is known, however, to be present in several crop plants including barley, wheat, soybeans and sweet potatoes. Best known is barley .beta.-amylase, due to its importance in brewing. (See, for example, Cook, 1962, Barley and Malt, Academic Press, New York and London; Briggs, 1978, Barley, Chapman & Hall, London; Pollock, 1979, Brewing Science Vol. 1, Academic Press; and Briggs, Hough, Stevens & Young, 1981, Malting and Brewing Science, Chapman & Hall.)
A special application for pure .beta.-amylase is the production of high maltose syrups having a very low glucose content. Such syrups offer distinct advantages in certain applications, for example, in confectionary applications.
The above-cited literature, its references and U.S. Pat. No. 3,492,203 disclose a plurality of methods for the separation of .beta.-amylase from cultivated plants or components thereof. A common feature of all these methods is that the starting material is initially ground or crushed, or that it comprises some mechanically separated component of grain, such as wheat bran. From these starting materials, .beta.-amylase is extracted with water, a buffer solution or the like. The obtained raw extract contains, together with .beta.-amylase, a number of other soluble grain ingredients, which must be removed. This leads to very arduous purification and concentration steps in the enzyme preparation.